Cell-based packet switching networks are becoming widely available. The use of small (i.e. "short") packets of information is preferred in modern digital networks because it enables the efficient mixing of synchronous and asynchronous information, thus providing cost-effective transport of digital voice, LAN data and video. Furthermore, short packets, also known as "cells", can be switched by integrated circuits, allowing quick and economical switching of data in broadband fiber optics networks. This concept is known in the telecom industry as "Asynchronous Transfer Mode" (ATM). ATM networks are commercially available. ATM protocols have been formalized by various international organizations, including the ITU and the ATM Forum. ATM networks were specified assuming the use fiber optics links for transmission. Due to the very low bit error rate of fiber optic links, ATM networks do not provide extra overhead services to guarantee end-to-end delivery of cells. Cells are routed through the network, but if an error occurs (or a buffer overflows), cells may be discarded. The simplicity of "best effort" cell delivery results in a fast and cost effective network.
Typical fiber networks consist of long-haul fiber links interconnecting ATM switches. These switches may be connected by fiber optics links to customer sites, such as office buildings and homes. In the customers' buildings there are network access nodes that combine and convert a user's information to ATM cells for transmission over the network.
Although fiber optics links are becoming the preferred medium for terrestrial links, they are not always available. City regulation, installation costs, long installation time and legal right-of way issues prevent some regions from installing fiber optics links. Some cities may have fiber optics links installed, but owned by a monopoly which a service provider may wish to bypass.
Digital microwave radio links can provide an alternative to fiber optics links between network access nodes and ATM switches. Frequency bands within the range of about 300 MHz to 60 GHz have been allocated for commercial communications. Some microwave links in the millimeter wave region are unlicensed or licensed for low usage fees by regulating governments. Microwave radio links then become a cost-effective and a timely solution to the fast deployment of telecommunication links. There is a drawback, however, to these microwave radio links. Digital microwave radio communication is prone to bit errors, especially under weather-induced fading conditions, such as rain. Some forward error correction, redundancy and retransmission protocol schemes have been devised to improve the performance of microwave links. The problem with these approaches is that they are not directly applicable for ATM traffic. Retransmission is unacceptable because of the delay it introduces. Forward error correction alone does not protect from antenna obstruction or antenna failure. Redundancy by parallel links is too costly and still prone to common link obstructions such as weather-induced signal degradation.